1. Field of the Invention (Technical Field)
The present invention relates to control of remelting processes such as vacuum arc and electroslag remelting processes. More specifically, the present invention discloses a system for dynamic control of melt rate and electrode position during remelting processes.
2. Background Art
Electroslag remelting (ESR) and vacuum arc remelting (VAR) are consumable electrode melting processes used in the production of premium grade superalloys as well as for remelting of various grades of steel. VAR is also used to process zirconium and aerospace titanium alloys. Both processes rely on electrical current as an energy source for melting. Both processes also entail flow of electrical current through a consumable metal electrode. In VAR, an electrical arc is established at the electrode tip which heats the tip causing it to melt. During ESR, the electrical current flows out the electrode tip through a slag bath in which its tip is immersed. The bath is heated resistively by the current causing it to be molten. This molten slag bath melts the electrode tip as the electrode is fed into it. Ingots are grown in both processes as the electrodes are consumed. Both processes utilize a water-cooled copper crucible for this purpose. The ingots partially solidify in their water-cooled crucibles as the grow, the metal remaining molten near the top where the energy inputs are located. Overall, ESR and VAR help to produce fully dense, defect-free, homogenous ingots having the appropriate chemistry, physical size, and grain structure.
The rate at which the electrode is consumed, typically specified in terms of weight melted per unit of time, is an important process parameter. Control of the melt rate generally allows for the production of higher quality ingots. Sometimes melt rate is controlled in an open-loop fashion whereby melting occurs according to a preset current schedule. This type of control, although relatively simple to implement, does not respond well to process disturbances. Of particular concern are disturbances in the relationship between melt rate and melting current. Some researchers have tried to remedy such situations through use of active, closed-loop, melt rate control (U.S. Pat. No. 4,141,754, entitled "Automatic Melt Rate Control System For Consumable Electrode Remelting," to Roberts, issued Dec. 26, 1978). Such dosed-loop approaches typically use a load cell transducer to continuously monitor electrode weight. Electrode weight data provide a basis for estimating melt rate. Melt rate estimates feed back to the controller whereby the electrical current is varied by an amount proportional to the error between estimated and set-point melt rates. This type of melt rate control operates best under steady-state conditions that use a constant set point. In essence, simple closed-loop control is effective during steady-state melting or relatively slow transient melting only. Simple dosed-loop control is limited primarily because load cell data are very noisy and require averaging over many (typically twenty) minutes before a reasonably accurate melt rate can be estimated. In general, commercial melt rate controllers use simple closed-loop controllers that are inherently rather sluggish and not capable of implementing an aggressive melt rate schedule. Again, such controllers operate best under steady-state conditions where the intended or set point melt rate is constant. Prior-art melt rate control is purely responsive, based on past information, and simply state that over the last measurement period, the melt rate was too high or low, and order a decrease or increase of the current by some amount.
VAR and ESR are inherently dynamic processes. Though steady-state melting is usually desired in the middle portion of the remelting process, the beginning and ending regions are highly dynamic by nature. Optimization of these processes requires that intelligent melt rate schedules be implemented in these regions and this, in turn, requires dynamic melt rate control. For example, steep, precisely controlled melt rate ramps may be beneficial during startup. One may wish to drop the melt rate from its initially high value at startup to the nominal melt rate over a time period of a few minutes or even seconds. If reliable melt rate estimates are delayed by ten minutes, as is the case with the current generation of melt rate controllers, this is an impossible task because the ramp would be over long before the system ever registered a response. If one simply steps the current down to the nominal value associated with the desired melt rate, uncontrolled dynamics are introduced which cause the system to overshoot the target and then recover only very slowly (usually a few hours). This behavior results from the long times required for the heat stored in the electrode to reconfigure itself to accommodate the new melt rate. Similar problems are encountered when the melt rate is disturbed from its desired value. An example would be a melt rate deviation due to a cracked electrode. The current generation of melt rate controllers respond only very slowly and incorrectly, allowing sever, uncontrolled melt rate excursions to occur.
Electrode position is another suitable control parameter for remelting processes. In most VAR processes, the electrode is positioned to maintain the proper electrode gap, defined as the spacing between the electrode tip and the molten surface of the ingot. In most controllers, the gap is estimated by monitoring the voltage or by counting "drip shorts"--the momentary shorting of the voltage that occurs when a molten droplet forming on the electrode tip bridges the gap between the electrode and the molten pool below. However, during some transient periods, drip short measurements prove inadequate for indicating electrode position and gap. Similarly, for ESR, the electrode is positioned to maintain an appropriate immersion depth--the depth that the electrode tip is immersed in the molten slag bath. For ESR processes, the voltage "swing" or the variance of the voltage over a period of time, is used to estimate the depth. Sole use of such voltage values leads to unacceptably large variations in the immersion depth during transient periods and produces undesirable process instabilities.
For the foregoing reasons, ESR and VAR simple dosed-loop controllers prove inadequate for implementing practically anything other than a constant melt rate.